Increased bone turnover occurs during normal preadult growth; destruction of bone from accidental, metabolic, or neoplastic causes; and as an effect of certain medications. For many years skeletal x-ray was the only clinical method used to detect bone change. Unfortunately, significant change could not be seen until about 50% of bone density was lost. Later, radionuclide bone scans supplemented x-ray, but bone scans were best suited to detect focal rather than generalized abnormality and were better able to detect an osteoblastic than an osteolytic process.

About the same time bone scans became important, it was found that a substance called hydroxyproline (part of the collagen and elastin component of skin, cartilage, and bone) could be used as an index of bone turnover since bone contains a large amount of metabolically active collagenous matrix. Hydroxyproline is a by-product of collagen metabolism, during which it is released into the blood and either catabolized in the liver or excreted in urine. There were a variety of problems associated with hydroxyproline assay. Either a collagen-free diet or an overnight fast and substitution of a hydroxyproline/creatinine ratio were required. There was a diurnal variation with maximum excretion between midnight and 8 A.M. and minimum between noon and 8 P.M. Assay methods were not standardized or completely satisfactory. Hydroxyproline excretion was used mainly to detect the presence of bone metastases (sensitivity, about 75%-80%; range, 36%-95%) and to monitor therapy; it never became popular.

More recently, proteins were found that specifically cross-link and stabilize collagen fibers in cartilage and bone; pyridinoline (PYD) is present in cartilage and bone while deoxypyridinoline (DPD) is present only in bone. Neither is influenced by diet. Both are released when bone matrix is dissolved as part of a resorptive process (either local or generalized; either an osteolytic or metabolic process; or active bone turnover). Therefore, PYD or DPD excretion increases in Paget’s disease, primary hyperparathyroidism, bone metastases, RA, osteomalacia, and osteoarthritis. PYD and DPD are excreted in urine without alteration by the liver. Analytic methods include high performance liquid chromatography and immunoassay. Both are currently being used in research centers primarily to detect bone loss in metabolic bone disease, especially osteoporosis.

In addition to metabolic turnover studies, bone mineral density is being measured by conventional x-ray methods, computed tomography, and radionuclide techniques; in each case, one or two small bone areas are evaluated and the results extrapolated to the skeleton as a whole. This has mainly been applied to evaluation of osteoporosis. Laboratory involvement in osteoporosis at present mainly is directed at excluding “secondary” etiologies. These are corticosteroid excess (Cushing’s syndrome or cortisol therapy), hyperthyroidism, myeloma, and possibly the uncommon cases of estrogen deficiency due to gonadal hormone deficiency. Screening tests for each are discussed in different chapters. In addition, serum calcium, phosphorus, and alkaline phosphatase are useful as a baseline and to (occasionally) detect diseases affecting bone (ALP elevated in 94% of osteomalacia).

Another marker for bone turnover is Gla protein (osteocalcin), the largest (20%) noncollagen protein of bone matrix. This substance is produced only by osteoblasts (and tooth-forming odontoblasts) and is excreted by the kidneys; there appears to be some breakdown in the kidneys. Serum bone Gla measured by radioimmunoassay was found to be increased in conditions associated with increased osteoblastic activity (e.g., Paget’s disease, osteomalacia, renal osteodystrophy, and osteoblastic bone metastases). However, there were some problems. Renal failure results in retention and increase of Gla in serum; there is relatively mediocre sensitivity in detection of skeletal metastases; and there were inconsistant results in conditions such as osteoporosis where the degree of bone turnover was relatively small. There is contradictory data on effects of age and female hormone changes. Bone Gla protein is vitamin K-dependent and is affected by thyroid hormone, parathyroid hormone, and growth hormone through their activity on bone metabolism. Estrogen and corticosteroids decrease bone Gla levels. To date, bone Gla protein assay has not become popular except in research centers. There is also a matrix Gla protein secreted by osteoblasts and found in bone and cartilage.